During processing of silver halide color photographic elements, the developed silver is oxidized to a silver salt by a suitable bleaching agent. The oxidized silver is then removed from the element in a fixing step.
The most common bleaching solutions are desirably acidic, and contain complexes of high valence metal ions, such as ferric ions, and various organic ligands. Both the metal ions and the chelating ligands present environmental concerns and are the subject of increasing regulatory scrutiny. Thus, a primary desire in this industry is to design bleaching compositions that are more compatible with the environment, with reduced use of high valent metal complex bleaching agents.
Bleaching solutions, such as those containing peroxide, or peracids such as persulfate, perborate, perphosphate, percarboxylic acid or percarbonate, as bleaching agents, offer an alternative to the ferric complex bleaching solutions. They are less expensive and present lower chemical and biological demands on the environment since their by-products can be less harmful. A number of such compositions have been explored, but to date they have not found wide acceptance in the trade for various reasons.
Various peroxide and peracid bleaching solutions are described, for example, in U.S. Pat. No. 5,318,880 (English et al), U.S. Pat. No. 5,464,728 (Szajewski et al), U.S. Pat. No. 5,508,151 (O'Toole et al) and U.S. Pat. No. 5,521,056 (Buchanan et al).
While persulfate bleaching agents have low environmental impact, they have the disadvantage that their bleaching activity is slow and thus require the presence of a special bleaching accelerator either in the processed material or in a processing bath (such as a bleach prebath) as taught by U.S. Pat. No. 5,318,880 (noted above). The most common bleaching accelerators have offensive odors.
Because hydrogen peroxide reacts and decomposes to form water, a peroxide based bleaching solution offers many environmental advantages over persulfate and ferric complex bleaching solutions. As a result, many publications describe peroxide bleaching solutions, including U.S. Pat. No. 4,277,556 (Koboshi et al), U.S. Pat. No. 4,301,236 (Idota et al), U.S. Pat. No. 4,454,224 (Brien et al), U.S. Pat. No. 4,717,649 (Hall et al), U.S. Pat. No. 5,550,009 (Haye et al), U.S. Pat. No. 5,541,041 (Haye), U.S. Pat. No. 5,641,615 (Haye et al), WO-A-92/01972 (published Feb. 6, 1992), WO-A-92/07300 (published Apr. 30, 1992) and EP 0 428 101A1 (published May 22, 1991). These compositions may comprise various amounts of chloride ions and have a pH in the range of 5 to 11.
Hydrogen peroxide bleaching compositions however, often cause physical defects in the processed photographic elements, such as blistering (or vesiculation), and suffer long term stability problems at certain acidity levels. In addition, peroxide is ineffective in oxidizing developed silver to silver halide at highly acidic pH values, and thus it cannot be used to directly replace current high valent metal ligand bleaching solutions.
There is a desire and need in the photographic industry to find solutions to all of these problems. Thus, there is a need to avoid the use of bleaching accelerators and high valent metal bleaching agents while providing an effective acidic bleaching environment.